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    <title>Document</title>
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    <script>
        console.log(Array.prototype);
        //原先的数组对象并没有sum方法
        Array.prototype.sum=function(){
            var sum=0
            for(var i=0;i<this.length;i++){
                sum+=this[i]
            }
            return sum
        }
        var arr=[1,2,3]
        console.log(arr.sum());
    </script>
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